The two pole wheels are in the form of radial plates, each including teeth which project from the radial plate substantially parallel to the rotor axis in the manner of claws. The teeth of one of the pole wheels are offset circumferentially with respect to those of the other pole wheel, so that the teeth of the two wheels inter-penetrate with each other. The rotor winding being supplied with direct current, the teeth of one of the pole wheels constitute south poles while those of the other wheel constitute north poles. When the teeth move past the stator poles by virtue of rotation of the rotor, they set up an induced alternating current in the stator winding.
When an alternator of this kind is operating, it is necessary to cool it in such a way that the whole of the machine remains at a limited temperature so as to ensure its best performance. It is known to effect cooling of such alternators by means of a fan which is fixed to the rotor and arranged at one end of the latter, usually outside the casing which encloses the stator and the rotor.
In order to improve the cooling effect, and thus to increase the output of such alternators, it is becoming increasingly common to fit the fans inside the casing itself, with the fans then being secured on to the radial plates of the pole wheels so that their cooling vanes lie facing the stator windings which emit a substantial amount of heat. Although this design gives improved cooling and therefore increased electrical output from the alternator, it does have the drawback that it also gives rise to noise due to turbulence in the air within the casing.
An acoustic study has been carried out, and has demonstrated that these noises were the result, in particular, of movement of the turbulent air in the immediate proximity of the radial plate of the fan, that is to say close to the outer periphery of the pole wheels lying facing the stator windings. This noise becomes even greater since such alternators are now required to operate at ever-increasing speeds, e.g. 1800 to 2000 r.p.m..
In order to reduce this noise effect, consideration has been given to modifying the design of the fans themselves, for example by providing them with circumferentially asymmetrical vanes. Although it is found that such an arrangement enables the noise level to be attenuated in general terms, this attenuation is not by any means sufficient to result in the alternator being capable of being regarded as silent.